Chemical & Engineering News reports that mauveine, a synthetic purple dye serendipitously invented by William H. Perkin and no longer commercially available, was used to color a 6d postage stamp in Great Britain from 1867-1880. The original research was conducted by Portuguese chemist João Ségio Sixas de Melo and published in Chemistry: A European Journal.
Thanks to Allen Hoffman for sending a clipping of the article for our archives.